🔌 Basic LV Electrical Engineer Interview Questions &
Answers
1.
What is low voltage (LV)?
Answer:
Low voltage refers to electrical systems that operate at 600 volts or less
(commonly 230V/400V in Saudi Arabia). These are used for lighting, power
outlets, and small equipment.
2.
What is the standard voltage in Saudi Arabia?
Answer:
- Single phase:
230V
- Three phase:
400V
- Frequency: 60 Hz
3.
What is the difference between single-phase and three-phase power?
Answer:
- Single-phase:
Has one live (L) and one neutral (N), used for homes.
- Three-phase:
Has three live wires (L1, L2, L3) and one neutral, used for commercial or
industrial loads.
4.
What is a circuit breaker?
Answer:
A circuit breaker is a safety device that disconnects power when there
is an overload or short circuit, protecting equipment and people.
5.
What is an MCB?
Answer:
MCB (Miniature Circuit Breaker) protects circuits against overload
and short circuit in low-voltage systems.
6.
What is an RCD or RCCB?
Answer:
RCD or RCCB (Residual Current Device) protects people from electric
shock. It trips when there is a leakage current to earth (like when someone
touches a live wire).
7.
Why is earthing important?
Answer:
Earthing provides a safe path for fault current to flow to the ground,
which prevents electric shock and equipment damage.
8.
What is the color coding of wires in Saudi Arabia?
Answer:
As per IEC standards:
- Brown
– Phase (L)
- Blue
– Neutral (N)
- Green/Yellow
– Earth (E)
9.
How do you test a cable before energizing?
Answer:
- Continuity Test
– Check if cable is connected end-to-end.
- Insulation Resistance Test (Megger) – Check insulation between conductors and earth.
- Polarity Test
– Make sure phase and neutral are not reversed.
10.
What tools do you use as an LV engineer?
Answer:
- Megger (for insulation)
- Clamp meter (for current)
- Multimeter (voltage/resistance)
- Phase sequence meter
- Crimping and stripping tools
11.
What is a distribution board (DB)?
Answer:
A DB (or panel) distributes electricity from the main supply to various
circuits in a building. It contains MCBs, RCDs, busbars, and wiring.
12.
What is the difference between load and current?
Answer:
- Load
is the total power used (measured in watts or kW).
- Current
is the flow of electricity (measured in amps).
13.
What is the formula for power?
Answer:
P=V×IP = V \times I
Where:
P = Power (watts)
V = Voltage (volts)
I = Current (amps)
14.
What is voltage drop?
Answer:
Voltage drop is the reduction in voltage along a cable run due to
resistance. It can cause poor performance of equipment.
15.
How do you prevent voltage drop?
Answer:
- Use larger cable size
- Reduce cable length
- Avoid sharp bends and bad joints
16.
What is a short circuit?
Answer:
A short circuit happens when a live wire touches neutral or earth
directly, causing high current and tripping breakers.
17.
What safety gear should an electrical engineer wear?
Answer:
- Safety helmet
- Safety shoes
- Insulated gloves
- Eye protection (goggles)
- Flame-resistant uniform
- Lockout/Tagout tags
18.
What is a multimeter and how do you use it?
Answer:
A multimeter measures voltage, current, resistance, and continuity. You
use probes to test terminals or wires in a circuit.
19.
How do you isolate power before working?
Answer:
- Switch off the breaker
- Apply LOTO (Lockout-Tagout)
- Test for voltage using a tester
- Confirm circuit is de-energized before touching
20.
What is your role as a junior LV engineer on-site?
Answer:
- Supervise cable pulling, DB installation, and
termination
- Ensure proper testing (IR, continuity)
- Follow safety rules
- Report progress and issues to supervisor
- Assist during inspections
21.
What is a capacitor and why is it used in LV systems?
Answer:
A capacitor stores electrical energy. In LV systems, it's used for power
factor correction—it helps reduce electricity bills and improves system
efficiency by reducing reactive power.
22.
What is the power factor?
Answer:
Power factor is the ratio of real power (kW) to apparent power (kVA).
- Ideal power factor = 1 (or 100%)
- Low power factor means more current is used, which
causes losses.
23.
What is the function of a contactor?
Answer:
A contactor is an electrically controlled switch used to control
large loads like motors, pumps, or lighting circuits. It is operated by a
control signal (often from a push button or timer).
24.
What is the difference between a relay and a contactor?
Answer:
- Relay
is used for low-power control (like signals, lights).
- Contactor
is for high-power switching (motors, heavy loads).
25.
What is an overload relay?
Answer:
An overload relay protects motors from drawing too much current for a
long time, which could damage the motor. It trips the circuit when overload
is detected.
26.
What are the main parts of an LV panel?
Answer:
- Incomer (main breaker)
- Busbars (distribute power)
- Outgoing MCBs
- Earth and neutral bars
- Control wiring
- Metering (optional)
27.
What is a busbar?
Answer:
A busbar is a metal bar (usually copper or aluminum) used inside
panels to distribute power from the main breaker to outgoing MCBs.
28.
What is the minimum insulation resistance value for a cable?
Answer:
Generally, it should be 1 MΩ or higher for LV circuits (tested at 500V
DC using a megger).
29.
What are the types of cables used in LV systems?
Answer:
- Single-core or multi-core
- PVC or XLPE insulated
- Copper or aluminum conductor
- Armored or unarmored (based on location)
30.
What is an IP rating in electrical equipment?
Answer:
IP = Ingress Protection rating. It tells you how well the equipment is
protected from dust and water.
Example:
- IP55: Protected against dust & water spray
- IP65: Dust-tight and water jet protected
31.
What causes an MCB to trip?
Answer:
- Overload
(too much current for too long)
- Short circuit
(direct connection of live and neutral)
- Sometimes, earth faults or loose connections
32.
What is the difference between fuse and MCB?
Answer:
- Fuse
melts and breaks the circuit when current is too high.
- MCB trips
(can be reset), more modern and reusable.
33.
What is a load schedule?
Answer:
A load schedule is a table showing all electrical loads in a system,
their power ratings, and breaker sizes—used for DB design and cable sizing.
34.
How do you calculate current from power?
Answer:
For single-phase:
I=PV×PFI = \frac{P}{V \times PF}
For three-phase:
I=P3×V×PFI = \frac{P}{\sqrt{3}
\times V \times PF}
Where:
- P = power in watts
- V = voltage
- PF = power factor (usually 0.8 for motors)
35.
How do you check phase sequence?
Answer:
Use a phase sequence meter:
- Connect the three probes to L1, L2, and L3.
- It shows the rotation (clockwise or anti-clockwise).
- Wrong phase sequence can cause motors to run in reverse.
36.
What is LOTO (Lockout-Tagout)?
Answer:
LOTO is a safety procedure where power is locked off and tagged
during maintenance to prevent accidental re-energizing.
37.
What is the minimum clearance required in front of an electrical panel?
Answer:
Usually 1 meter in front of the panel, as per IEC and SEC
guidelines, for safe operation and maintenance.
38.
What is a short-to-earth fault?
Answer:
It occurs when the live conductor touches the earth—it causes the RCD to
trip and can be dangerous without proper earthing.
39.
What is the purpose of a DB schedule?
Answer:
It lists:
- Circuit numbers
- Load names (like lighting, sockets)
- Cable sizes
- MCB ratings
Helps during installation, testing, and troubleshooting.
40.
What are some common safety rules on site?
Answer:
- Wear PPE (helmet, gloves, shoes)
- Don’t touch live wires
- Use insulated tools
- Isolate and tag before work
- Follow supervisor’s instructions
- Keep panels locked when not in use
41.
What are the types of earthing systems used?
Answer:
- TN-S:
Separate neutral and earth conductors
- TN-C-S:
Combined neutral and earth up to a point
- TT: Earth
from local earth rod
- IT:
Isolated neutral (used in hospitals, etc.)
Most Saudi projects follow TN-S
or TN-C-S, depending on SEC/Aramco guidelines.
42.
What is the standard frequency in Saudi Arabia?
Answer:
60 Hz (unlike Europe which uses 50 Hz)
43.
What is the function of a neutral wire?
Answer:
Neutral wire completes the circuit and carries return current back to
the source. It is at or near ground potential.
44.
What are the types of circuit breakers used in LV systems?
Answer:
- MCB (Miniature Circuit Breaker) – up to 100A
- MCCB (Molded Case Circuit Breaker) – up to 1600A
- ACB (Air Circuit Breaker) – 800A and above (main panel)
- RCCB/ELCB
– for earth leakage protection
45.
What is a feeder pillar?
Answer:
A feeder pillar is an outdoor electrical cabinet that distributes power
to different areas. It contains breakers and terminals for cable connections.
46.
What is a trunking and conduit system?
Answer:
- Trunking:
Rectangular or square enclosure used for routing multiple cables (surface
mounted).
- Conduit:
Round pipes (PVC or GI) used to protect and route cables inside walls or
ceilings.
47.
What is diversity factor in electrical design?
Answer:
It accounts for the fact that not all loads run at full capacity all the
time. Helps in proper sizing of main breakers and cables.
48.
What is a harmonics problem in LV systems?
Answer:
Harmonics are distortions in electrical waveforms caused by non-linear
loads (like VFDs, UPS). They can overheat transformers, trip breakers, and
damage equipment.
49.
What is an LV switchgear?
Answer:
Switchgear is a combination of devices like breakers, disconnectors,
contactors, and busbars used to control and protect LV circuits.
50.
How do you test an RCD?
Answer:
- Use the test button on the RCD (should trip
immediately).
- Or use an RCD tester that applies test current
to measure trip time.
51.
What is the minimum depth for underground cable laying?
Answer:
Typically, 600 mm for LV cables. It must be covered with:
- Sand layer
- Cable warning tape
- Concrete tiles (in some areas)
52.
What is a cable gland?
Answer:
A cable gland secures the cable at entry points of panels or DBs,
providing:
- Mechanical support
- Dust and moisture sealing
- Earthing (in case of armored cable)
53.
What is the function of a surge protection device (SPD)?
Answer:
SPD protects equipment from voltage spikes (e.g., lightning or switching
surges). It diverts excess voltage to the earth.
54.
What is meant by “de-rating” a cable?
Answer:
De-rating means reducing the cable's current-carrying capacity due to factors
like:
- High ambient temperature
- Grouping of cables
- Underground installation
Used to avoid overheating.
55.
What is loop impedance?
Answer:
Loop impedance is the total resistance of the circuit from the source to the
fault point and back, important for fault current calculation and breaker
selection.
56.
What is difference between isolator and breaker?
Answer:
- Isolator:
Manually operated, used only when circuit is off.
- Breaker:
Can operate under load and protects against faults.
57.
What is a VFD (Variable Frequency Drive)?
Answer:
VFD controls motor speed by changing supply frequency. Used for energy
saving and soft starting of motors (e.g., in HVAC or pumps).
58.
What is meant by phase-to-phase and phase-to-neutral voltage?
Answer:
- Phase-to-phase (L–L):
400V (3-phase systems)
- Phase-to-neutral (L–N): 230V
59.
What are the causes of low insulation resistance?
Answer:
- Damaged or old cable
- Moisture in cable
- Poor insulation quality
- Overheating or mechanical damage
60.
What is your first step before starting any electrical work?
Answer:
- Isolate power
- Lockout/Tagout (LOTO)
- Verify with tester
- Only then start work
.jpg)
0 Comments